1. Field of the Invention
This invention relates to a writing method for a magnetic random access solid state memory (MRAM) and a magnetic random access memory, more particular a writing method for a MRAM which are preferably usable, in a harsh condition, for example, in an aerospace system.
2. Description of the Prior Art
Recently, semiconductors have achieved a very high recording density at a high speed. In contrast a MRAM using a magnetization in a magnetic film, which was proposed a long time ago, cannot have a recording density competing with that of semiconductor RAM memories because MRAM because of delay of the magnetic film technology compared with that of the semiconductor films. Recently, however, micro-pattering technique of magnetic thin films has developed rapidly and much attention is being paid to the essential stability (radioactive resistance and thermal stability) of MRAM mainly in industries of the United States.
FIG. 1 is a schematic view explaining writing method of conventional MRAM. The conventional MRAM, as shown in FIG. 1, has electrodes connected to vertical wires Lx1-Lx5 and lateral wires Ly1-Ly5 that are patterned orthogonal. On each crossing point of the wires there are magnetic film component as indicated with a1-e5.
Writing this MRAM is performed by reversing the magnetic flux of the film C3, for example. The flux reversal is made by flowing current to the wire Lx3 and Ly3 from an external power source, giving rise to a magnetic field strong enough for flux reversal. For reading the written information the magnetization direction of the film C3 is detected by a magnetic sensor positioned near it.
MRAM has intrinsically a higher speed of writing than that of semiconductors. Moreover, MRAM is composed of metallic layers such as magnetic films. This leads to an advantage of easier micro patterning and, furthermore, metallic materials have a conduction electron density higher than that of carriers in semiconductors by several orders of magnitude. This leads to a high potential of ultra-high integration of elements.
However, writing of MRAM developed so far is carried out with a strong magnetic field by current pulses in the crossing wires as described above. Consequently, the current results in ohmic heating in the wires giving rise to temperature increase of the element. So, as a memory density increases, the ohmic heat brings about destruction of the element or unstable operation. This limits the memory density of MRAM.
Moreover, concentration of a strong magnetic field within a limited area near the magnetic film is extremely hard. This also limits integration density of MRAM.
It is an object of the present invention to provide a new writing system for MRAM and a new MRAM that eliminates the problem describe above.
This invention relates to a writing method for MRAM comprising the steps of:
forming, on a substrate, a lattice shaped underlayer of a ferroelectric material having a piezoelectric effect,
forming, on the crossing points of the underlayer, films of a magnetic material having a magnetoelastic effect,
and applying a certain voltage to a given row and column of the underlayer and thereby, reversing the magnetic flux of the film on the crossing point, whereby writing of MRAM is performed.
This invention also relates to MRAM comprising of a substrate, an underlayer made of a ferroelectric material having an piezoelectric effect, magnetic films made of a magnetic material having a magnetoelastic effect, the underlayer being formed in a lattice shape on the substrate, magnetic films being formed on the crossing points of the lattice underlayer.
The inventors have studied intensively to figure out a new writing method for MRAM and a new MRAM to put the writing method into practice. As a result, they found that when an underlayer made of a ferroelectric material with an piezoelectric effect is formed in lattice shape on a substrate, and a magnetic film with a magnetoelastic effect is formed on each crossing point of rows and columns of the underlayer, the lattice shaped underlayer works as a writer.
The writing process of MRAM of the present invention will be explained hereinafter, based on a conceptual view referring to the method. FIG. 2 is a conceptual view to explain the writing method for MRAM of the present invention.
The MRAM shown in FIG. 2 has a lattice shaped underlayer P made of a ferroelectric material with a piezoelectric effect on a substrate not shown in the figure. Then, the memory has magnetic films A1-E5 made of a magnetic material having a magnetoelastic effect on each crossing point of the columns X1-X5 and the rows Y1-Y5 of the underlayer P.
For example, to perform writing by reversing the flux of the film C3, a voltage is applied to the column X3 and to the row Y3 of the underlayer P. Consequently, a large stress is given rise on the crossing point of X3 and Y3 by the piezoelectric effect of the ferroelectric material constituting the underlayer P. The large stress is transmitted to the film C3 on the crossing point, leading to distortion of the film C3.
Since the magnetic films of the present MRAM are made of magnetic material with high magnetoelastic effect, the stress of the film C3 is converted to magnetoelastic energy. Then, the energy is converted into the magnetic energy by reversal of the magnetization.
Herein, only the crossing point of the column X3 and the row Y3 has a strong stress and other crossing points do not receive any stress strong enough to give rise to flux reversal in the films on them.
Writing for a MRAM of the present invention is carried out by taking advantage of the piezoelectric effect of the underlayer made of a ferroelectric material having a high piezoelectric effect and the magnetoelastic effect of films having a high magnetoelastic effect. When a voltage is applied to the underlayer, no current flows in the underlayer because it is highly resistive.
Thus, ohmic heat is negligible in this invention and the damage to the wirings in the conventional MRAM is removed, which leads to improved stability of operation for this new MRAM. Consequently, the new writing method removes limitations on high density MRAM caused by energy dissipation in the wirings.